Treatment of catalyst particles

ABSTRACT

The invention relates to processes and apparatus for the elutriation of particles of a Ziegler-Natta type alpha-olefin polymerisation catalyst, based on transition metal compound(s) and preferably also comprising magnesium compounds, by means of an elutriation liquid which is a non-polar liquid, for example a hydrocarbon chemically inert in relation to the catalyst. A preferred process comprises the following stages:- preparation of a homogeneous suspension of the catalyst in the elutriation liquid. and- elutriation of the catalyst by means of the suspension and the elutriation liquid in one or two elutriation columns (F1),(F2), separating the large and fine catalyst particles respectively.The apparatus comprises a tank (M2) for the preparation of the catalyst suspension and the elutriation column or columns (F1)(F2).The invention also relates to elutriation of Ziegler-Natta catalyst particles using a non-polar elutriation liquid and wherein a special polar aprotic additive is employed, for example an organo-metallic compound.

This is a division of application Ser. No. 002,162, filed Jan. 12, 1987now U.S. Pat. No. 4,714,553.

The present invention relates to the improvement of Ziegler-Natta typealpha-olefin polymerisation catalysts based on one or more transitionmetal compounds.

More particularly, the invention relates to processes and apparatus forthe liquid elutriation of particles of alpha-olefin polymerisationcatalyst, and makes it possible to obtain catalysts having a relativelynarrow particle size distribution. These catalysts can be used in thepolymerisation of alpha-olefins under low pressure to produce polyolefinpowders themselves possessing a relatively narrow particle sizedistribution.

Ziegler-Natta catalyst systems comprise one or more transition metalcompounds belonging to Groups IV, V or VI of the Periodic Table ofElements, and at least one organo-metallic compound of metals of GroupsII or III of this Table. Generally, in the art of Ziegler-Nattapolymerisation the transition metal-containing component of the catalystsystem is referred to as the "catalyst", whilst the organometalliccompound is referred to as the "co-catalyst". This terminology will beused in the present specification. The catalysts are frequently solidcompounds consisting of titanium halides, preferably associated withcompounds of magnesium, having a relatively high activity in thepolymerisation of alpha-olefins, which advantageously makes it possibleto avoid at the end of polymerisation the stage of removing thecatalytic residues present in the polymers. The co-catalysts generallycomprise organo-aluminium or organo-zinc compounds which are liquid orgaseous under the normal polymerisation conditions.

When the polymerisation of alpha-olefins is conducted in suspension in aliquid hydrocarbon medium, or in the gas phase, the solid polymer beingformed develops inside and on each particle of catalyst. Provided thatthis development of polymer occurs in a uniform manner, the particlesize distribution of the produced polymer particles remains similar tothat of the catalyst, whilst the average particle size of the polymerparticles increases as the reaction continues.

Highly active catalysts based on transition metal and magnesiumcompounds are generally prepared by contacting at least one transitionmetal compound with metallic magnesium or a magnesium compound, forexample an organomagnesium compound, magnesium oxide, magnesiumhydroxide, a magnesium alcoholate, magnesium hydroxychloride ormagnesium chloride, such contacting being performed for example bychemical reaction, impregnation or grinding. The produced catalystsoften have a comparatively broad particle size distribution associatedwith a high content of fine particles. This can cause problems ingas-fluidised bed polymerisation of olefins due to the fact that thefine particles are easily entrained by the fluidisation gas into partsof the apparatus not intended for carrying out on the polymerisationreaction.

Moreover the presence of fine particles of polyolefin, for example thosepossessing a diameter of less than 50 or 100 microns, provides a dustexplosion risk during handling of the powders, can lead to losses ofpolyolefins during their conversion and can diminish the flowability ofthe polyolefin powders, thus hindering the feed to conversion machinery.

Large particles of polyolefin, for example those possessing a diameterup to several millimeters, provide difficulties in the pneumaticconveying of the powders.

It has been suggested that the fine and large catalyst particles shouldbe removed by sieving or by elutriation using a flow of gas or liquid.However, having regard to the fineness of the catalyst particles, such asize segregation or "selection" operation has hitherto not been achievedunder satisfactory industrial conditions.

In general, processes are known for segregating particles which use thephysical principle of separating the particles by their differences indensity. However, such processes cannot be applied to Ziegler-Natta typecatalysts, whose particles are differentiated not by density, but bysize.

Processes for segregating particles are also known which use thephysical principle of separating particles by their differences in size.Such a segregation can be performed in an elutriation column in which anelutriation liquid flows in an ascending stream at a substantiallyconstant speed and in laminar flow conditions--i.e., without turbulence.The segregation is performed by applying the physical principle of thevariation in the speed at which the particles settle in the elutriationliquid in relation to their sizes. The largest particles settle towardsthe bottom of the column in relative contraflow with the elutriationliquid, while the finest particles are entrained to the top of thecolumn with the elutriation liquid. Attempts to apply this type ofelutriation technology to the segregation of Ziegler-Natta catalystparticles have encountered a number of difficulties, particularly due tothe relatively low density of the catalyst particles, the relativelysmall average particle sizes involved and difficulties in relation tothe selection of a suitable elutriation liquid. The elutriation liquidis desirably chemically inert in relation to the catalyst, and forpractical reasons this restricts the choice of elutriation liquidsessentially to non-polar liquids, for example, liquid hydrocarbons.However, it is found that the use of such non-polar liquids aselutriation liquids can cause aggregation or agglomeration of thecatalyst particles making it difficult to achieve satisfactory particlesegregation.

It is an object of the present invention to provide an improved processand apparatus for segregating Ziegler-Natta catalyst particles by liquidelutriation.

Accordingly the present invention provides a process for elutriation bya liquid of solid particles of a Ziegler-Natta catalyst to obtainseparation into at least two portions which differ in average particlesize characterised in that in a preliminary stage (stage 1) a suspensionof the solid catalyst particles is prepared in the elutriation liquidwhich is a non-polar liquid chemically inert in relation to thecatalyst, at a concentration in the range 20 to 150 grammes per liter,and further in that the process comprises one or more of the followingdefined stages M and N, wherein stage M is a process for separatinglarge particles comprising introducing the catalyst suspension at a flowrate Q1 into a vertical elutriation column F1 having a height H', at alevel between H'/2 and the bottom of the column, introducing theelutriation liquid at a flow rate R1 into the column F1 at a level lowerthan that of the introduction of the catalyst suspension, the liquidbeing caused to flow in the column in an ascending stream substantiallyunder laminar flow conditions, withdrawing from the top of the column F1a catalyst suspension substantially free from large particles and,withdrawing from the bottom of the column F1 a catalyst suspensionmainly comprising large particles and wherein stage N is a process forseparating fine particles comprising introducing the catalyst suspensionat a flow rate Q2 into a vertical elutriation column F2 of height H at alevel above H/2 and below 7H/8, introducing the elutriation liquid at aflow rate R2 into the column F2 at a level below H/2, the liquid beingcaused to flow in the column in an ascending stream and substantiallyunder conditions of laminar flow, withdrawing from the top of the columnF2 elutriation liquid charged with fine catalyst particles andwithdrawing from the bottom of the column F2 catalyst particlessubstantially freed from fine particles.

The laminar flow conditions mentioned in stages M and N above ischaracterised by a Reynolds number (Re) lower than 2000 and preferablylower than 1000. In the case of cylindrical columns of circular sectionthe Reynolds number is dimensionless and equal to:

    Re=(4·Q·n)/(pi·d·v)

where

Q=flow rate of the liquid,

n=density of the liquid,

d=diameter of the column, and

v=viscosity of the liquid.

and pi is the universal constant 3.14159...

In the variants of the process according to the invention, therefore,the catalyst suspension in the elutriation liquid prepared during state1 can be subjected to an elutriation operation enabling either the largeor fine particles of the catalyst to be separated. The suspension canalso be subjected to a double elutriation operation enabling both thelarge and fine catalyst particles to be separated simultaneously, thelarge particles being separated in the elutriation column F1, preferablybefore the fine particles are separated in the elutriation column F2.

The catalyst suspension fed to the elutriation is preferably welldispersed and of substantially uniform constitution throughout.

The invention also relates to an apparatus for performing theelutriation process according to the invention and comprising:

a tank M2 for the preliminary preparation of a catalyst suspension in anelutriation liquid and having means adapted to maintain uniformity inthe suspension,

a vertical elutriation column F1 having a height H' and a diameter D'such that the ratio H'/D' is equal to or greater than 5, such columnhaving: (a) a tube for the introduction of catalyst suspension preparedin the tank M2 or of the catalyst suspension substantially freed fromfine particles coming from the outlet of the top of column F2 as thecase may be, at a level lying between H'/2 and the bottom of the column;(b) a tube for the introduction of the elutriation liquid at a levellower than that of the introduction of the catalyst suspension;

(c) an outlet from the top of the column for catalyst suspensionsubstantialy freed from large particles; and (d) an outlet from thebottom of the column for catalyst suspension comprising mainly largeparticles, and/or

a vertical elutriation column F2 having a height H and a diameter D suchthat the ratio H/D is equal to or greater than 10, the column having:(a) a tube for the introduction of the catalyst suspension prepared inthe tank M2 or of the catalyst suspension substantially freed from largeparticles coming from the outlet at the top of the column F1, as thecase may be, such tube being disposed at a level above H/2 and below7H/8; (b) a tube for the introduction of the elutriation liquid at alevel lying below H/2; (c) an outlet from the top of the column forcatalyst particles substantially freed from fine particles.

The apparatus can also advantageously comprise means for separating thecatalyst particles and the elutriation liquid, for example, adecantation or filtration device, or a liquid cyclone, and also meansfor recycling the elutriation liquid freed from the catalyst particlesto the feed to the column F1 and/or F2.

The invention is further illustrated with reference to the accompanyingdrawing.

FIG. 1 is a simplified diagram of a liquid-phase elutriator according tothe invention, in which elutriation can be carried out on the one handof the fine particles and on the other hand of the large particlespresent in powdery catalyst.

The process may be implemented in an apparatus such as that shown inFIG. 1 and operating in the following manner:

A uniform suspension is produced of the solid catalyst in theelutriation liquid. This suspension, maintained in agitation in the tankM2, is pumped by the pump G2 so as to feed the column F1 in whichelimination of the large particles is carried out; these latter are thenconveyed to the tank M5. The suspension freed from large particles andleaving column F1 at the top then feeds column F2. The columns F1 and F2are fed with decanted elutriation liquid coming from the tank M4 bymeans of the pump G3.

The elutriation liquid leaving the column F2 at the top entrains thefine particles separated from the elutriation liquid by means of thehydrocyclone Z. The fine particles separated are introduced into thetank M5 where decanting of the separated particles is performed, whilstthe elutriation liquid separated in the hydrocyclone Z is sent to thetank M4.

The elutriated catalyst leaves the column F2 at the bottom. It iscollected in the tank M3 in the form of a concentrated suspension.

In order to obtain good particle size separation of the fine particlesand/or large particles in the catalyst the following provisions arepreferred:

the ratio between the height H' and the diameter D' of the elutriationcolumn F1 is equal to or greater than 5, preferably equal to or greaterthan 10;

the ratio between the height H and the diameter D of the elutriationcolumn F2 is equal to or greater than 10, preferably about 20;

the point at which the catalyst suspension is introduced into theelutriation columns F1 and F2 has an influence on the quality ofseparation; if H is the height of the elutriation column F2 adapted toseparate the fine particles, the catalyst suspension is advantageouslyintroduced above H/2, preferably at 3H/4 or above, and below 7H/8;

moreover, in the case of the elutriation column F1 adapted to separatethe large particles and having a height H', the catalyst suspension ispreferably fed at a level lying between H'/2 and the bottom of thecolumn, preferably lying between H'/4 and the bottom of the column;

also advantageously the elutriation liquid is introduced into thecolumns F1 and F2 at a point lying below the point where the catalystsuspension is introduced, preferably at a point such that the distanceseparating the points of introduction of the elutriation liquid and thecatalyst suspension is equal to or greater than H'/8, preferably equalto or greater than H'/4 in the column F1, and equal to or greater thanH/4, preferably equal to or greater than H/2 in the column F2, since ithas been found that a sufficiently large distance separating the twopoints of introduciton into a column enables the dispersion of theparticles in the elutriation liquid to be improved and thereforeimproves the quality of the selection; the introduction of theelutriation liquid into the column F1 is therefore performed at a levellying preferably below H'/4; similarly, the elutriation liquid isadvantageously introduced into the column F2 at a level preferably lyingat H/4 or there below;

the concentration of the catalyst suspension in the elutriation liquidprepared in the tank M2 must be relatively low, lying more particularlybetween 20 and 150 g/l, and preferably between 40 and 100 g/l; such aconcentration enables the quality of the particle size selection to beimproved and the elutriated catalyst yield to be enhanced;

the catalyst concentration in the elutriation columns is advantageouslylow; more particularly, in the column F1 it lies between 10 and 100 g/l,preferably between 30 and 70 g/l, while in column F2 it lies between 2and 60 g/l, preferably between 5 and 30 g/l; in these conditions thelarge and fine catalyst particles can be efficiently separated and thenreadily eliminated; the ratio between the flow rates R1/Q1 in the columnF1 therefore advantageously lies between 0.2 and 5, preferably between0.5 and 2; similarly, the ratio between the flow rates R2/Q2 in thecolumn F2 is advantageously between 0.3 and 6, preferably between 0.5and 4;

the liquid used for elutriation must not impair the catalysts;preferably it is a dry liquid aliphatic hydrocarbon freed from oxygen,for example, n-heptane or n-hexane.

The process according to the invention can be performed by continuousintroduction of the catalyst suspension and elutriation liquid into theelutriation column or columns (permanent continuous conditions), or bycontinuous introduction for a predetermined period of the catalystsuspension into the elutriation column or columns and continuousintroduction of the elutriation liquid until the whole of the catalystcharge has been elutriated (non-permanent continuous conditions).

Depending on the catalysts, one proceeds to a simple selection (fine orlarge) or to a double particle size selection (fine and large), eachtype of selection involving the elutriation characteristics peculiar toit (dimension of columns, throughput of solvent). These characteristicsare determined experimentally within the limits disclosed hereinbefore.

It has been observed that the particle selection is improved when theelutriation is conducted in the presence of a small quantity of one ormore special additives.

Accordingly, the present invention further comprises a process forelutriation of particles of a Ziegler Natta-type catalyst comprisingperforming the elutriation using a non-polar liquid elutriation mediumin the presence of a polar aprotic compound, preferably anorganometallic compound. The Ziegler Natta-type catalyst comprises acompound or compounds of one or more transition metals selected frommetals of Groups IV, V or VI of the Periodic Table (Mendeleer). TheZiegler Natta-type catalyst preferably comprises one or more transitionmetal compounds associated with or in chemical combination with one ormore magnesium compounds. The preferred polar aprotic compounds aresoluble in the elutriation liquid. Most preferably they areorgano-metallic compounds of the type used as co-catalysts inZiegler-Natta catalyst systems; these co-catalysts generally consist oforgano-metallic compounds of metals belonging to Groups II or III of thePeriodic Table, especially of organoaluminium, organozinc ororganomagnesium compounds comprising at least one aluminium/carbon bond,such as trialkylaluminiums, halides or alcoholates of alkylaluminium,dialkylzinc and dialkylmagnesium. Preferred compounds aretriethylaluminium, triisobutylaluminium, tri-n-hexylaluminium,tri-n-octylaluminium, diethylaluminium chloride, ethylaluminiumsesquichloride, ethoxydiethylaluminium or diethylzinc. These compoundsare preferably used at concentrations in the range 0.1 and 100millimoles per litre of elutriation liquid.

The polar aprotic compound employed as the special additive in theprocess of the present invention is preferably selected from compoundswhich do not deleteriously affect the catalyst. In the case that theadditive is an organometallic compound, some reduction and/or activationof the catalyst particles can take place. The use of the defined polaraprotic compounds can lead to improvements in the quality of theparticle size selection, the yield of elutriated catalyst and thereproducibility of the operations.

The catalysts employed in the processes of the invention preferablyconsist essentially of halogenated compounds of transition metalsbelonging to Groups IV, V or VI of the Periodic Table of Elements andcompounds of magnesium and optionally with aluminium compounds.Preferred catalysts have the general formula:

    Mg.sub.m Al.sub.n M(OR.sub.1).sub.p X.sub.q D.sub.r

in which M is an atom of titanium and/or vanadium, R₁ is an alkyl groupcomprising 2 to 14 carbon atoms, X is an atom of chlorine and/orbromine, D is an electron donor compound comprising at least one atom ofoxygen, or sulphur, or nitrogen, or phosphorus, but not comprising anatom of active hydrogen,

wherein;

m lies in the range 1.5 to 50, preferably 2 to 10,

n lies in the range 0 to 2, preferably 0 to 1,

p lies in the range 0 to 3,

q lies in the range 4 to 100, preferably 5 to 27, and

r lies in the range 0 to 60, preferably 0 to 20.

These catalysts can be obtained by various processes in themselvesknown, in particular those according to which a magnesium compound, forexample a magnesium halide, is ground in the presence of at least onehalogenated compound of a transition metal and optionally an electrondonor compound, or else a magnesium compound is precipitated at the sametime as one or more halogenated transition metal compounds, optionallyin the presence of an electron donor compound.

The catalysts can be obtained, for example, by reacting anorganomagnesium compound with a halogenated transition metal compoundtaken at its maximum valency in the presence of a halogenating agent andoptionally an electron donor compound D, having the same definition asabove, chosen for example from amongst amines, amides, phosphines,sulphoxides, sulphones, ethers and thio-ethers. This reaction isadvantageously performed using these compounds in quantities such thatthe molar ratio of the quantity of organomagnesium compound to thequantity of halogenated transition metal compound is greater than 1, theexcess of organomagnesium compound being decomposed by the halogenatingagent so that no substantial quantity of magnesium-carbon bonds remains.

The catalysts may also be obtained by reacting magnesium metal with analkyl halide in the presence of a halogenated transition metal compoundtaken at its maximum valency and optionally an electron donor compound Dhaving the same definition as above. This reaction is advantageouslyperformed using a quantity of magnesium metal such the molar ratio ofthe quantity of magnesium metal to the quantity of halogenatedtransition metal compound is greater than 1, and a quantity of alkylhalide such that after the reaction substantial quantities of compoundscomprising a magnesium-carbon bond are no longer present.

The processes of the present invention are particularly useful for sizeseparation by liquid elutriation of Ziegler-Natta catalyst particleshaving a density in the range 1 to 2, preferably in the range 1.2 to1.6, and having an average particle size in the range 10 to 100 microns.Such particles can have irregular shapes and rough surfaces.

The catalysts obtained by elutriation according to the present inventioncan be used in processes of polymerisation or copolymerisation ofalpha-olefins, especially in gas-phase polymerisation orcopolymerisation processes, and in particular in a fluidised bed.

The invention is illustrated by the following examples, in which Example1 is given by way of comparison:

EXAMPLE 1 (COMPARISON)

(a) Preparation of the catalyst

A 1-liter glass reactor, provided with a mechanical stirrer, a refluxcondenser and a heating or cooling device is filled with dry nitrogen;there are introduced into it successively at ambient temperature:

12.15 g (500 m.Moles) of magnesium in powder form

23.75 g (125 m.Moles) of titanium tetrachloride

92.5 g (1 Mole) of n-butyl chloride

n-heptane, to make up the volume to 600 ml.

After the addition of 1.26 g of iodine, the reaction medium is heatedwith stirring to 75° C., so as to cause the reaction to commence. Thereaction begins slowly after about an hour and a half and the reactionmedium is maintained at 75° C. for 3.5 hours. The resultant brown/blackprecipitate is washed several times with heptane. The catalyst Aobtained has the following composition by weight:

Ti : 10.3%

Mg : 19.2%

C1 : 70.5%

(b) Polymerisation of ethylene

Into a 5-liter stainless steel reactor provided with mechanical stirringthere are introduced under an atmosphere of nitrogen 2 liters ofn-heptane at ambient temperature. After heating the n-heptane to 70° C.,one introduces:

0.46 g (4 m.Moles) of triethylaluminium

a quantity of catalyst corresponding to 1 milligram atom of titanium.

When the reaction medium has been heated to 75° C., hydrogen isintroduced into it until a pressure of 0.6 MPa is obtained, thenethylene at a throughput of 160 g/hr.

After 7 hours of polymerisation, 1100 g of polymer are collected, thetitanium content of which is 34 parts per million by weight (ppm).

The particle size distribution of the polymer is shown in Table I.

EXAMPLE 2

(a) Preparation of the elutriated catalyst

1 Kg of the catalyst A is prepared using the same conditions as inExample 1(a). The catalyst is subjected to liquid elutriation using thefollowing conditions:

    ______________________________________                                        apparatus          as shown in FIG. 1, but                                                       omitting the column (F1)                                   diameter of the column (F2)                                                                      70 mm                                                      height of the column (F2)                                                                        1600 mm                                                    elutriation operation                                                                            continuous non-permanent                                   nature of elutriation liquid                                                                     n-heptane                                                  level of introduction of the                                                                     150 mm above the bottom                                    elutriation liquid into the column                                            throughput of elutriation liquid                                                                 15 liters per hour                                         (R2)                                                                          catalyst concentration in the                                                                    100 millimoles of                                          catalyst suspension                                                                              titanium/liter-i.e 60 g of                                                    catalyst per liter                                         throughput of catalyst suspension                                                                4 liters per hour                                          (Q2)                                                                          catalyst concentration in the                                                                    13 millimoles of titanium                                  column (F2)        per liter-i.e. 7.8 g of                                                       catalyst per liter                                         level of introduction of the                                                                     1200 mm above the bottom                                   catalyst suspension into the                                                  column (F2)                                                                   yield of elutriated catalyst                                                                     50%                                                        (based on titanium contained                                                  in the catalyst)                                                              hourly production  0.2 millimoles of titanium/                                                   hour-i.e. 0.12 Kg of                                                          catalyst per hour.                                         ______________________________________                                    

The catalyst B obtained after elutriation has the same chemicalcharacteristics as catalyst A.

(b) Polymerisation

Example 1(b) was repeated using catalyst B instead of catalyst A.

1100 g of a polyethylene powder are obtained, the particle sizedistribution of which is shown in Table I.

It is found that the polyethylene obtained from catalyst B has a contentof particles with dimensions below 350 microns which is appreciablylower than that of the polyethylene obtained from catalyst A, thisdecrease being particularly significant for particles with dimensionsbelow 50 microns.

EXAMPLE 3

(a) Preparation of the elutriated catalyst

1 Kg of catalyst prepared in accordance with Example 1(a) is subjectedto liquid elutriation under the following conditions:

    ______________________________________                                        apparatus         as shown in FIG. 1, omitting                                                  column (F1)                                                 diameter of column (F2)                                                                         70 mm                                                       height of column (F2)                                                                           1600 mm                                                     elutriation operation                                                                           continuous non-permanent                                    nature of elutriation liquid                                                                    n-heptane                                                   height of introduction of the                                                                   150 mm above the bottom                                     elutriation liquid into the                                                   column (F2)                                                                   throughput of elutriation liquid                                                                15 liters/hour                                              (R2)                                                                          nature of additive contained in                                                                 tri-n-octylaluminium                                        the elutriation liquid                                                        additive concentration in the                                                                   0.7 millimole/liter                                         elutriation liquid                                                            catalyst concentration in the                                                                   100 millimoles of titanium/                                 catalyst suspension                                                                             liter-i.e. 60 g of catalyst                                                   per liter                                                   throughput of catalyst suspension                                                               4 liters/hour                                               (Q2)                                                                          catalyst concentration in the                                                                   17 millimoles of titanium/                                  column (F2)       liter-i.e. 10.4 g of catalyst                                                 per liter                                                   level of introduction of the                                                                    1200 mm above the bottom                                    catalyst suspension into the                                                  column (F2)                                                                   yield of elutriated catalyst                                                                    65%                                                         (based on titanium contained                                                  in the catalyst)                                                              hourly production 0.26 millimoles of titanium                                                   per hour-i.e. 0.16 Kg of                                                      catalyst per hour                                           ______________________________________                                    

The catalyst C obtained after elutriation has the same chemicalcharacteristics as catalyst A.

(b) Polymerisation

Example 1(b) is repeated using catalyst C instead of the catalyst A.

1100 g of a polyethylene powder are obtained, the particle sizedistribution of which is shown in Table I.

It is found that the polyethylene obtained from catalyst C has a contentof particles with dimensions below 350 microns which is appreciablysmaller than that of the polyethylene obtained from catalysts A and B,this decrease being particularly great for particles of dimensions below50 microns.

It is also noted that the yield of catalyst C is appreciably higher thanthe yield of catalyst B obtained in Example 2.

EXAMPLE 4

In order to determine the reproducibility of the elutriation operationsdescribed in Examples 2 and 3, each of these examples is reproduced 10times from the same catalyst A, and one determines:

the mean percentage by weight of particles of polyethylene withdimensions below 160 microns and the corresponding standard deviation,

the mean yield of elutriated catalyst and the corresponding standarddeviation.

The results obtained are set out in Table II.

The standard deviation,

    √Σ(deviation).sup.2,

that is to say the square root of the sum of the squares of thedeviations by comparison with the mean result, gives an indication ofthe dispersion of the results obtained and consequently of thereproducibility of the tests.

It is found that the standard deviations obtained according to Example 3are smaller than those obtained in Example 2, both as regards thepercentage of particles with dimensions below 160 microns and thecatalyst yield, which indicates that the conditions of Example 3 provideappreciably more reproducible results than those of Example 2.

                                      TABLE I                                     __________________________________________________________________________    COMPARISON OF THE PARTICLE SIZES OF THE POLYETHYLENE OBTAINED                 WITH THE NON-ELUTRIATED CATALYST (CATALYST A) AND THE POLYETHYLENE            OBTAINED WITH THE ELUTRIATED CATALYST (CATALYSTS B AND C)                                      Aperature of sieves (microns)                                                 above                                                                             710-                                                                             500-                                                                             350-                                                                             250-                                                                             160-                                                                             125-                                                                             80-                                                                              50-                                                                              below                                             1000                                                                              1000                                                                             710                                                                              500                                                                              350                                                                              250                                                                              160                                                                              125                                                                              80 50                               __________________________________________________________________________    Percentage by weight                                                                     Catalyst A                                                                          1    7 16 23 21 16 5  6.5                                                                              2.5                                                                              2                                of particles retained                                                                    Catalyst B                                                                          2   10 24 26 17 12 3.5                                                                              4  1  0.5                              between two sieves                                                                       Catalyst C                                                                          3   15 24 26 16 10.5                                                                             2  2.5                                                                              1  0                                __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________    Example 2, reproduced 10 times                                                                         Example 3, reproduced 10 times                       % by weight of particles with                                                                     Catalyst                                                                           % by weight particles with                                                                   Catalyst                              dimensions below 160 microns                                                                      Yield                                                                              dimensions below 160 microns                                                                 Yield                                 __________________________________________________________________________    Mean 9              50   5.5            65                                    Result                                                                        Typical                                                                            3              10   1.5             5                                    deviation                                                                     __________________________________________________________________________

I claim:
 1. A process comprising: elutriating by a liquid, solidparticles of a Ziegler-Natta catalyst to obtain separation into at leasttwo portions which differ in average particle size, by providing in apreliminary stage (stage 1) a suspension of the solid catalyst particlesprepared in the elutriation liquid which is a non-polar liquidchemically inert in relation to the catalyst, at a concentration in therange 20 to 150 grammes per liter, and further providing in saidprocess, one or more of the following defined stages M and N,whereinstage M is a process for separating large particles comprising,providing a vertical elutriation column F1 having a height H',introducing the catalyst suspension at a flow rate Q1 and at a levelbetween H'/2 and the bottom of the column, introducing the elutriationliquid at a flow rate R1 into the column F1 at a level lower than thatof the introduction of the catalyst suspension, causing the liquid toflow in the column in an ascending stream substantially under laminarflow conditions, withdrawing from the top of the column F1 a catalystsuspension substantially free from large particles and, withdrawing fromthe bottom of the column F1 a catalyst suspension mainly comprisinglarge particles and wherein stage N is a process for separating fineparticles comprising, providing a vertical elutriation column F2 ofheight H, introducing the catalyst suspension at a flow rate Q2 and at alevel above H/2 and below 7H/8, introducing the elutriation liquid at aflow rate R2 into the column F2 at a level below H/2, causing the liquidto flow in the column in an ascending stream and substantially underconditions of laminar flow, withdrawing from the top of the column F2elutriation liquid charged with fine catalyst particles and withdrawingfrom the bottom of the column F2 catalyst particles substantially freedfrom fine particles.
 2. A process as claimed in claim 1, characterisedin that the ratio of the flow rates R1/Q1 in the elutriation column F1is between 0.2 and
 5. 3. A process as claimed in claim 1, characterisedin that the ratio of the flow rates R2/Q2 in the elutriation column F2is between 0.3 and
 6. 4. A process as claimed in claim 1, characterisedin that the catalyst concentration in the elutriation column F1 isbetween 10 and 100 g/l and the catalyst concentration in the elutriationcolumn F2 is between 2 and 60 g/l.
 5. A process as claimed in claim 1,characterised in that the elutriation liquid is an aliphatichydrocarbon.
 6. A process as claimed in claim 6 wherein the ZieglerNatta-type catalyst comprises one or more transition metal compoundsassociated with or in chemical combination with one or more magnesiumcompounds.